This invention relates to handover control for multi-mode wireless devices: that is to say, devices which are able to support two or more wireless interfaces. Some of these devices are able to hand over a data connection or voice call from one wireless interface to another without the session or call being dropped. Such arrangements are particularly useful for handsets capable of operating on both localised high bandwidth systems such as WLANs (Wireless local area networks) and public cellular networks which have greater geographical penetration but more limited facilities for example in bandwidth.
The decision as to when to hand over is typically taken by the device, by monitoring one or more parameters such as signal strength on both the current interface and the candidate for handover, so that if a predetermined threshold is reached indicating the availability of a better interface than the one currently in use, a handover from the current wireless interface to the other, better, one is initiated. It is known for handover to be initiated only if the difference in signal strength exceeds a predetermined value, to avoid too-frequent handovers when the signal strengths are similar. It is also known for the thresholds to be weighted, for example so that a cheaper or feature-rich service is favoured over another service.
Typically a single parameter is measured and a handover threshold selected, though in some cases multiple parameters are monitored and an algorithm based upon the parameters and one or more thresholds used to determine whether to hand over or not.
It is known to modify criteria for handover between one base station and another in the same network in response to handover failures. In this situation handover is readily initiated because the handset and base station are continuously exchanging data relating to the availability of neighbouring base stations. However, if it is desired to switch between one network and another, only the handset has all the necessary information as the base stations are not in co-operation with each other—indeed it is more likely to be in each operators' commercial interests to seize control of the mobile unit from the other networks.
Network operators spend a lot of time, effort and money optimising the handover thresholds so that handsets maximise their coverage for the optimum network without the reliability of handovers dropping too much for any usage case. Setting a handover threshold so that a handover is attempted too late may result in the handover proving unsuccessful. However, the selection of the threshold for handover is more critical when switching between networks, because attempting a handover earlier than necessary may be disadvantageous, particularly if the second interface is more expensive to connect to, or lacks capabilities available on the first, or if the advantage it offers is temporary because of variations in signal quality on one or both networks.
Current practice bases handover decisions on a fixed set of thresholds for parameters which are combined in an algorithm to decide whether to initiate handover or not, as shown in the flow-chart of FIG. 1. In that figure, after a call is initiated (1) on a first network, an assessment (2) is made periodically to determine whether to hand over. If the decision is made to do so, the handover (3) takes place. If the handover is unsuccessful the call will fail (4), otherwise it continues (5) until the next periodic assessment (2). Note that the review period (6) may vary according to the network to which the handset is currently connected. United Kingdom Patent Application GB2337184 discloses a mobile unit which applies a default threshold condition, from which it may depart if conditions improve, but to which it reverts if contact is lost.
According to the invention, there is provided a wireless communications device having a handover processor for controlling the handover of connection of the device from a first communications network to a second communications network, having a signal quality assessment system for determining values for one or more measure of signal quality currently available from at least one of the communications networks, and determining whether the measure or measures of signal quality meet threshold values set for that network, a handover processor for initiating a handover attempt if the threshold values are met, and a threshold setting unit for adjusting one or more of the threshold values according to the success or failure of the handover attempt.
The invention also provides a process for controlling handover of a connection between two co-operating devices between a first communications network and a second communications network, in which values for one or more measure of signal quality are determined in at least one of the communications networks, the measure or measures of signal quality are compared with threshold values set for that network. A handover attempt is initiated if the threshold values are met, and one or more of the threshold values are adjusted according to the success or failure of the handover attempt.
Where one of the communications networks is to be used in preference to the other, but the other is of ubiquitous availability, it is only necessary to measure the threshold values for the preferred system. To avoid too-frequent failure of the handover process, adjustment of the thresholds in the event of a successful handover may be more limited, in frequency or magnitude, than adjustments made in consequence of a handover failure.
This invention can be used for both handset-initiated handovers and for network-initiated handovers.
This invention differs from existing systems by introducing an iterative process to learn the optimum settings for handover thresholds from experience. The advantage of this is that it enables the handover thresholds to be optimised for the actual equipment in use and the way it is used, rather than making use of a conservative threshold for all devices and environments.
This invention would significantly reduce the existing costs expended by network operators or handset manufacturers in determining the optimum handover thresholds for installing in the network or handsets. Instead, the handset or network automatically optimises coverage in use, for many different usage cases.